The present invention relates generally to a method of removing a glass coating that surrounds a metal wire core in a manner that protects the surface of the metal core from damage. In particular, the invention relates to the removal of the silica-based glass coating that surrounds an amorphous metal microwire.
Metal microwires can be produced by the Taylor-Ulitovsky production process (reference U.S. Published Patent Application 2006/0130995 A1 to Adar et al.). In this process, an alloy melt is rapidly solidified in a softened glass sheath. The presence of the softened glass sheath dampens instability in the alloy melt and promotes the formation of a glass-coated microwire with a uniform diameter and a smooth metal/glass interface. A glass-coated microwire with an amorphous metal core can also be produced by the Taylor-Ulitovsky method, in which a glass tube and the desired metal are brought into a high-frequency induction field. The metal is melted, and its heat softens the glass tube, so that a thin metal-filled capillary is drawn from the softened glass tube. The metal-filled capillary enters a cooling zone in a superheated state where it is rapidly cooled, such that the desired amorphous or micro-crystalline (i.e., grain size less than about 1 micrometer), micro-structure is obtained. Rapid cooling is typically required to obtain amorphous and micro-crystalline micro-structures. The rate of cooling is not less than 104 degrees C./sec and preferably is 105-106 degrees C./sec. The amorphous or micro-crystalline structure is controlled by choice of amorphous alloy, cooling rate, nature of the cooling liquid, location of the cooling stream, dwell time in the cooling stream and degree of super-heating and super-cooling.
For some applications it is important to subsequently remove the glass coating from the microwire. One exemplary application is the use of the microwire as an anode wire in a proportional radiation counter or neutron detector. In this application, charged particles must be able to make electrical contact with the bare metal surface of the wire. Removal of the glass-coating is desirable for the operation of the detector because: 1) incident electrons caused by ionization of the gas in the detector chamber should be free to impact the anode wire surface in order to produce a detector signal, and 2) the ends of the anode wire should make an electrical connection with the ends of the detector. During operation of the detector a radially symmetric electric field is caused to exist between the anode wire and the cathode shell of the detector. Symmetry of this field and consistency of the field gradient requires an anode wire with a smooth surface and a constant (i.e., uniform) diameter. Furthermore, some applications may require position-sensitive detectors in which the ionization event is localized by measuring the difference in arrival times of the detector signal at each end of the detector assembly. Accuracy of the position measurement depends on the anode wire having a constant resistance along its length. Therefore, the diameter of the anode wire must be constant along the length of the detector.
It is well-known in the art that hydrofluoric acid (HF) is effective at dissolving silica-based glass compositions. Hydrofluoric acid dissolves glass by reacting with silicon dioxide (SiO2), the major component in most glasses. However, the dissolution process cannot be well-controlled. When using hydrofluoric acid to remove the glass coating from a wire it is difficult to avoid etching and pitting the metal core. An etched and pitted metal core no longer exhibits a smooth surface and uniform diameter, which is detrimental to many applications (e.g., neutron detection). Therefore, a need exists for a method to controllably and quickly remove the glass coating without damaging the metal core of a glass-coated wire while maintaining the wire's uniform diameter.